Apparatus and method for replaying decoded instructions

Abstract

An arrangement and method for decoding coded instructions and playing and replaying decoded instructions to a machine. The arrangement has a source of coded instructions. Connected to this coded instruction source is a decoder for receiving and decoding the coded instructions and for outputting the decoded instructions to a machine. A silo is connected to the output of the decoder and siloes and outputs the decoded instructions to the machine. The outputting of the decoded instructions to the machine are switched between the silo and the decoder, so that the machine receives the siloed decoded instructions. By siloing and then replaying already decoded instructions at the time of a trap occurrence, a speed increase is achieved, since the instructions which are in the trap shadow do not have to be decoded again.

What is claimed is: 
     
       1. A method of replaying instructions to a computer said instructions being decoded by a decoder, and said instructions occurring during a trap shadow, which is a time between an instruction which has caused a trap occurrence and issuance of a trap handler address, the method comprising the steps of: siloing the decoded instructions in the sequence that they are output by the decoder, for a period of time at least equal to said trap shadow;   issuing a trap handler address;   freezing further decoding of succeeding instructions by the decoder upon issuance of the trap handler address;   replaying said siloed decoded instructions to the computer upon issuance of the trap handler address; and   unfreezing the decoder to permit the decoder to continue decoding of the succeeding instructions after the siloed decoded instructions have been replayed.   
     
     
       2. The method of claim 1, further comprising the step of selecting to be output to the computer: the siloed decoded instructions upon issuance of the trap handler address; and   the succeeding instructions after unfreezing of the decoder and decoding of the succeeding instructions.   
     
     
       3. The method of claim 2, wherein the step of freezing the decoder includes freezing a clock to the said decoder. 
     
     
       4. An apparatus for decoding coded instructions and playing and replaying decoded instructions to a computer, the arrangement comprising: a source of at least one coded instruction;   means for issuing a trap handler address;   a decoder coupled to said coded instruction source for receiving and decoding the coded instructions and for outputting the decoded instructions to the computer;   means for continuously siloing the decoded instructions coupled to an output of said decoder, said means arranged to output the decoded instructions to the computer; and   means for switching between said means for siloing and said decoder the outputting of the decoded instructions to the computer,   said means for switching to output the decoded instructions from said decoder to the computer prior to the issuance of a trap handler address, and to output the siloed decoded instructions from said means for continuously siloing upon the issuance of a trap handler address by said means for issuing a trap handler address.   
     
     
       5. An apparatus according to claim 4, wherein said means for switching includes a multiplexer with first and second inputs coupled to respective outputs of said decoder and said means for siloing, and an output coupled to the computer, said multiplexer being responsive to multiplexer control signals indicating a trap in the computer has occurred so as to select outputting to the computer the decoded instructions in said means for siloing, and to select outputting to the computer the decoded instructions from said decoder when the multiplexer control signals do not indicated occurrence of a trap. 
     
     
       6. An apparatus according to claim 5, further comprising means for holding said decoder and said coded instruction source from decoding instructions until said means for siloing has completed outputting said siloed decoded instructions to the computer, in response to multiplexer control signals indicating a trap in the computer has occurred. 
     
     
       7. The apparatus of claim 6, wherein said means for siloing is a series of stage registers which holds each said decoded instruction for a period of time equal to the trap shadow. 
     
     
       8. The arrangement of claim 7, wherein said means for holding includes means for temporarily stopping clocks in said decoder and said coded instruction source. 
     
     
       9. The arrangement of claim 8, further comprising means for generating said multiplexer control signals and said hold control signals in response to recognition of said trap occurrence.